Electrophilic Substitution in Alkenes by the Example of Bromination of Vinylporphyrins

Abstract

The bromination of a carbon–carbon double bond is a classic reaction, the mechanism of which has been thoroughly studied for a long time. However, it has been found that the behavior of certain porphyrin substrates does not follow generally accepted concepts. Instead of bromine addition to the double bond, these substrates undergo substitution of a hydrogen atom by a bromine atom. Using the methods of low-temperature NMR spectroscopy and quantum chemical calculations, the mechanism of the bromination reaction of vinylporphyrins has been established. It has been demonstrated that the influence of a porphyrin substituent on the double bond leads to a change in the mechanism from electrophilic addition Ad_E, which is typical for alkenes, to electrophilic substitution S_E, which is characteristic of aromatic compounds but not alkenes. This fact significantly challenges the well-established classical concept that alkenes do not undergo electrophilic substitution reactions. The obtained results of mechanistic studies have not only fundamental scientific value, but also practical application: using the bromination reaction studied, it is possible to perform the CH-functionalization of vinyl substrates in a single step with high efficiency. As a result of this functionalization, corresponding bromovinyl (2-bromovinyl) derivatives are obtained, which are valuable electrophilic synthons— building blocks for catalytic reactions of carbon–carbon (cross-coupling) and carbon–heteroatom (substitution) bond formation.